ORGANIC COMPOUNDS OF PLANTS l8l 



periment? (8) What does this experiment show in regard to the 

 properties of cellulose ? 



Starch 



234. Occurrence. Starch is found most abundantly in 

 the seeds, roots, and tubers of plants, being stored up in 

 those parts which are concerned with new growth. 

 During growth, starch is produced in the leaves of all 

 green plants ; at maturity, it is stored in the seed or tuber. 

 It is present in the plant cells as granules which have 

 regular organized forms. 



235. Physical Properties. The starch granules from 

 a given cereal are always constant in form and physical 

 properties. Each grain is composed of overlapping layers 

 which can be observed under the microscope. The walls 

 of the layers are composed of a material called ' ' starch 

 cellulose ' ' ; between the walls is the pure starch known 

 as granulose. All starch grains have a somewhat similar 

 general structure. Starch is insoluble in cold water, be- 

 cause the walls of starch cellulose prevent the water from 

 dissolving the pure starch. In hot water some of the 

 granulose is dissolved and a paste is formed. Pure dry 

 starch is tasteless and odorless. Starch is exceedingly 

 hydroscopic, and commercial starch always contains from 

 10 to 12 per cent., or more, of water .^ A food which 

 contains a large amount of starch will vary in moisture 

 content and weight according to the hydroscopicity of the 

 air. The starch grains obtained from different cereals 

 and food products vary in form according to the source 

 from which they are obtained. Wheat starch is circular 



